A rare third head of the biceps femoris in the posterior thigh.

The occurrence of a third head of the biceps femoris is very rare. We encountered the case of a 90-year-old Japanese male cadaver with a third head of the biceps femoris in the posterior aspect of the thigh during dissection at Aichi Medical University in 2016. It originated from the proximal part of the femur and fused with the muscle belly between the long and short heads of the biceps femoris. Additionally, three muscle tendons were connected to the gluteus maximus. To the best of our knowledge, this is the first report on the third head of the biceps femoris demonstrating two origins, i.e., the proximal part of the femur and the insertion tendon of the gluteus maximus. Moreover, the third head, as well as the short head, of the biceps femoris was innervated by the muscular branch of the common peroneal nerve. Based on the origin and innervation, it can be believed that the third head of the biceps femoris is analogous to its short head and is related to the tenuissimus, a phylogenetic remnant. Therefore, we concluded that this third head is an intermediate muscle type of the tenuissimus and short head of the biceps femoris.